PART I. GENERAL ACCOUNT 



The function of the tentacular apparatus 



The tentacles undoubtedly perform some function which is very important 

 in the life of the animal. The structure of the tentacular crown varies enor- 

 mously in different forms (p. 25), whilst all the remaining organization of the 

 body is distinguished by its comparatively great constancy. The tentacular 

 apparatus appears to be the key organ system in the evolution of pogono- 

 phores and gives, therefore, a whole series of important systematic pointers 

 for the separation of families and genera. 



In spite of all the variation in numbers of tentacles and in the morphology 

 of the crown (p. 27), the tentacular apparatus presents a fundamental unity. 

 Thus in all forms except Siboglinum, the tentacles lie parallel to one another 

 enclosing [if there are enough of them] an intertentacular cavity. Even in the 

 unitentaculate Siboglinum [and in the bitentaculate genera] an intertentacular 

 cavity of a sort may be present, for in life, and often in fixed material too, 

 when the tentacle is retracted into the tube it is wound into a tight corkscrew 

 (Fig. 56). [In the bitentaculate genera the two tentacles are often found 

 twined round each other in a multiple helix]. 



The form of the intertentacular cavity may vary. Thus in Polybrachia for 

 the greater part of their length the tentacles lie parallel in concentric circles 

 between which lie concentric cavities (Fig. 57). In Lamellisabella and Spiro- 

 brachia, which possess a more regular tentacular apparatus, the tentacles are 



tdl 



Fig. 57. Transverse section of tentacular crown of Polybrachia annulata. 



cav - intertentacular cavity; cil - ciliated band; сое - coelomic canal of tentacle; nt - tentacular 

 nerve; pi- pinnule; tdl - left dorsal tentacle ; tvl - left ventral tentacle; va - afferent vessel; 

 ve - efferent vessel. (After Ivanov, 1959b.) 



